Throughout this application, various references are cited in parentheses to describe more fully the state of the art to which this invention pertains. The disclosure of these references are hereby incorporated by reference into the present disclosure.
Thick film dielectric structures as exemplified by U.S. Pat. No. 5,432,015 provide for superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to thin film electroluminescent (TFEL) displays. The thick film dielectric structure also enhances the amount of charge that can be injected in to the phosphor film to provide greater luminosity than TFEL displays.
Full colour thick film dielectric electroluminescent displays as is described in the Applicant's U.S. Patent Publication No. 2004/0135495 employ a high luminance blue phosphor material to directly illuminate blue sub-pixels and colour conversion materials to down-convert the blue light to red or green light for the red and green sub-pixels. The blue phosphor material is typically europium activated barium thioaluminate. In the Applicant's U.S. Patent Publication No. 2006/0017381 a thin vacuum deposited aluminum oxide layer is provided positioned directly under and in contact with the phosphor layer to enhance performance and stability.
Aluminum oxide barriers are also disclosed in the prior art as a barrier layer for electroluminescent displays. For example Japanese patent application 2003-332081 discloses an aluminum oxide layer disposed between the thick dielectric layers and the phosphor layer in a thick dielectric electroluminescent device. In the disclosed device a zinc sulfide layer is placed between the upper most aluminum oxide dielectric layer and the thioaluminate phosphor layer. The zinc sulfide layer functions as part of the phosphor layer in that electron injection for light emission occurs at the interface between the aluminum oxide layer and the zinc sulfide layer. The zinc sulfide layer inhibits sulfur loss from the thioaluminate material.
Aluminum oxide layers are also known to be used in organic electroluminescent devices where such layers are provided adjacent to a phosphor or substrate as described for example in U.S. Pat. Nos. 4,209,705, 4,751,427, 5,229,628, 5,858,561, 6,113,977, 6,358,632 and 6,589,674 as well as in U.S. 2003/0160247 and U.S. 2004/0115859.
These aforementioned developments provide thick film dielectric electroluminescent displays that fully meet the luminosity and colour spectrum capability of cathode ray tube (CRT) based television. However, it is still desired to further improve the operating stability to more fully meet television product specifications.